1. Field of the Invention
The invention relates to a roll, in particular a middle roll of a calender, with a roll jacket that surrounds an interior space. The invention further relates to a calender with a roll stack that features at least two middle rolls, at least one of which features a roll jacket that surrounds an interior space.
2. Discussion of Background Information
Calenders and middle rolls of the type in question are used to glaze paper or cardboard webs. These webs are thereby guided through nips that are formed between adjacent rolls and in these nips they are acted on with increased pressure and as a rule also with increased temperature. The nips are thereby embodied as so-called “soft” nips that are limited by a hard roll with smooth surface and a soft roll. The hard roll is made of metal as a rule, whereby the surface is also made of metal. The “soft” roll interacting with it features a comparatively softer surface. As a rule, this surface is made of a plastic coating that can be applied to a roll body, whereby the roll body itself can likewise be made of metal.
The roll stack normally features two end rolls, at least one of which, but usually both of which, are embodied as sag (deflection) compensation rolls. The sag compensation rolls feature a circulating roll jacket that can be acted on with the aid of hydraulic support elements such that a desired deflection line, preferably an extended line, is produced in the nips. The middle rolls located between the two end rolls are formed in a less costly manner. As a rule they comprise a roll jacket that surrounds an interior space. The interior space is closed at the faces by roll plugs.
After a certain operating time it can regularly be observed with such calenders that a barring formation occurs. Barrings are stripes that run crosswise to the travel direction of the web. As soon as these stripes become visible, the paper or cardboard web is broke that has to be disposed of.
The mechanisms causing this barring formation have not yet been finally resolved. It is assumed that it is a matter of self-excited vibrations that make the soft rolls “polygonal” through material wear over the course of time, or to put it more simply, that impress a wave pattern on the surface of the soft rolls. A roll stack that is formed of several rolls, has a plurality of natural frequencies. This does not refer to the natural frequencies of the individual rolls per se, such as, e.g., natural frequencies in bending, but the natural vibration forms that result from the vibrating roll masses on the spring and damper systems of the interposed plastic coatings.
An operating calender produces exciter forces, the frequencies of which are composed of the multiples of the rotational speeds of the roll. These exciter forces can be created by non-homogeneities, anisotropies or geometrical distortions (out of roundnesses). Likewise, fluctuations in the paper thickness of the paper or cardboard web entering the calender can excite the roll stack. A paper web entering the calender is still relatively rough before the glazing process. Moreover, a paper or cardboard web is not normally free from fluctuations in basis weight or thickness. When an excitation frequency, however it is caused, coincides with a natural frequency of the roll stack, the vibration system responds with enlarged vibration amplitudes. Due to the plurality of possible exciters and the plurality of possible natural vibration forms, resonance points usually cannot be bypassed by structural devices. Normally the vibration system is also so strongly damped and the exciter forces are so small that the resulting vibration movements are not directly disruptive. However, over a shorter or longer period these vibration movements are impressed in the plastic coatings of the elastic rolls, which leads to the barring formation described above after a certain operating duration. Usually several days or weeks elapse before this manifestation has developed so much that it disrupts the production process. The roll then has to be dismantled and reworked, which means considerable expense. Reworking, for example, can entail the roll being ground into a round shape again.